J-pole Antenna

The J-pole antenna, also called the Zepp' antenna (short for Zeppelin), was first invented by the Germans for use in their lighter-than-air balloons. Trailed behind the airship, it consisted of a single element, one half wavelength long radiator with a quarter wave parallel feedline tuning stub. This was later modified into the J-pole configuration, which became popular with amateur radio operators because it is effective and relatively simple to build.

The J-pole antenna is an end-fed omnidirectional dipole antenna that is matched to the feedline by a quarter wave transmission line stub. Matching to the feed-line is achieved by sliding the connection of the feedline back and forth along the stub until a VSWR as close as possible to 1:1 is obtained. Because this is a half-wave antenna, it will exhibit gain over a quarter-wave ground-plane antenna. The J-pole is somewhat sensitive to surrounding metal objects, and should have at least a quarter wavelength of free space around it. The J-Pole is very sensitive to conductive support structures and will achieve best performance with no electrical bonding between antenna conductors and the mounting structure.

A well known variation of the J-pole is the Slim Jim antenna, which is related to the J-pole the way a folded dipole is related to a dipole. The Slim Jim is one of many ways to form a J-Pole. Invented by Fred Judd (G2BCX), the name was derived from its slim construction and the J type matching stub (J Integrated Matching).

Both antennas should ideally be fed with balanced line, however a coax feed line may be used if a balun is added. Commonly, a choke balun is used, or an air transformer, using about five turns of coax. Typical construction materials include copper pipe, ladder line, or twin-lead. Coax can be used to match the J-pole as somewhere between the closed circuit and open circuit of the stub an exact 50 ohm impedance match exists.

The J-pole design functions well when fed with a balanced feed (via balun, transformer or choke) and no electrical connection exists between its conductors and surrounding supports. A common approach extends the conductor below the bottom of the J-pole resulting in additional and undesirable RF currents flowing over every part of the mounting structure. This modifies the far field antenna pattern typically, but not always, raising the primary lobes above the horizon reducing antenna effectiveness for terrestrial service. J-pole antennas with electrical connection to their supports often fare no better, and often much worse, than the simpler Monopole antenna.